Declassified protocol of the meeting of the "Interim Committee" on nuclear weapons which occurred on May 31, 1945. At this meeting, leading physicist Arthur Compton spells out that the Manhattan Project has thus far not managed to purify plutonium, and he projects that this will take two more years from the time of the meeting, with a third year required to obtain plutonium "in volume." He is not contradicted by any of the other physicists in attendance, including Robert Oppenheimer, Ernest Lawrence, and Enrico Fermi.
The conclusion is unavoidable that no plutonium bombs could have been detonated in Alamogordo and in Nagasaki mere weeks after this meeting. The full context is discussed in my book Hiroshima Revisited.
This PDF file used to be hosted at the National Security Archive, but it has been purged from there and is now found at this URL only by the Wayback Machine. In order to preserve this historically important document, it is posted here.